Silicon carbide powder finds wide usage in the manufacture of products suitable for refractory and engineering applications. Silicon carbide powder is useful for the preparation of products suitable for refractory and engineering applications.
It is known in the art to prepare silicon carbide powder by firing an intimately mixed green mixture of fine silica (SiO2) with carbon (C) under flowing argon gas atmosphere by following the equation as follows:SiO2+3C=SiC+2CO  (1)
Rice husk has conventionally been used as the source of fine silica (Lee et al. in Am. Ceram. Bull., Vol. 54, No. 2, pp. 195-98, 1975 titled “Formation of silicon carbide from rice hulls”). Krishnarao et al. (Ceram. Inter., Vol. 18, No. 4, 1992, pp. 243-49, titled “Distribution of silica in rice husks and its effects on the formation of silicon carbide”) study and describe the importance of distribution of silica in the starting material and the role of catalyst. A maximum yield of 60% SiC is the theoretical amount that is producible when rice husk is used as starting material.
Guterl et al. (J. Eur. Ceram Soc., vol. 19, No. 4, 1999, pp. 427-32, entitled “SiC material produced by carbothermal reduction of a freeze gel silica-carbon artefact”) teaches the fineness of both silica and carbon initial particle size. It is taught that monodispersed, extremely fine-sized silica powder with mean particle size ˜25 nm was effective for silicon carbide preparation. In this study, a sol-gel route was used to prepare the extreme fine starting silica powder. Cervic et al. (Ceram. Inter., Vol. 21, No. 4, 1995, entitled “A comparison of sol-gel derived SiC powders from Saccharose and activated carbon”) teach the importance of an extremely fine sized starting carbon with specific surface area of >950 m2g−1. In both above cases, a firing temperature of 1550° C. was required.
The requirement of further high firing temperature of 1550° to 1800° C. is reported in Martin et al. (J. Eur. Ceram Soc., vol. 18, No. 12, 1998, pp. 1737-42, titled “Synthesis of nanocrystalline SiC powder by carbothermal reduction”). Hanna et al. (Brit. Ceram. Trans. J., Vol. 84, No. 1, 1985, pp. 18-21, titled “Silicon carbide and nitride from rice hulls-III: Formation of silicon nitride”) teaches the use of a source of iron in the starting composition under a flow of ammonia gas in place of argon/nitrogen gas and above 1350° C. silicon carbide. A maximum of 90% of the starting silica could be reduced up to 1500° C. resulting in a mixture of silicon carbide and silicon nitride when an amount >6 wt. % iron was used in the starting mixture.
The drawbacks of the above processes are manifold. Firstly, extremely fine grain sized silica is required which has to be prepared following the sol-gel technique resulting in enhanced costs. Secondly, the use of large excess of carbon in the starting mixture involves an additional firing at above 500° C. in air of the post-reacted product where the unreacted carbon has to be burnt off. Additionally, the failure of achieving carbidation up to the theoretical value signifies that some residual silica remains in the product where no other phase appears in the reaction product. The residual silica may be harmful in the ultimate use of the material. The use of iron produces silicon carbide along with silicon nitride where an addition of ˜7 wt. % iron is required in case ammonia is the reacting gas.
The major drawbacks of the above noted hitherto known prior art processes are:
1. The starting silica particle size should be extremely small with surface area of the powder at least greater than 150 m2/g which is produced following a very expensive sol-gel process.
2. The starting carbon particle size should also be extremely fine with surface area of the powder preferably greater than 150 m2/g.